What is the point of parenting mesh?
Parenting provides an important convenience in organizing assemblies of objects - as mentioned in the commentary to your question.

But I guess the most important distinction between joining or grouping objects and parenting them is that, as far as transforms are concerned, the parent-child relationship is one-way. Children inherit the transformations of their parent - but a parent does not inherit the transformations of its children. This is vital to animation and rigging.

Consider a fairly simple animation of, say, the Earth orbiting the Sun, and the Moon orbiting the Earth (even without rotating at different speeds on their own axes) .. try it without parenting, and you'd soon be convinced of the need for it. There would be a lot of maths to do :)